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In the Amazon basin, about 16 percent of the original forest area has been cleared, and about 30 to 50 percent of cleared land is estimated now to be in some stage of secondary forest succession following agricultural abandonment. Here we use forest age chronosequences to demonstrate that young successional forests growing after agricultural abandonment on highly weathered lowland tropical soils exhibit conservative N-cycling properties much like those of N-limited forests on younger soils in temperate latitudes. As secondary succession progresses, N-cycling properties recover and the dominance of a conservative P cycle typical of mature lowland tropical forests re-emerges. These successional shifts in N:P cycling ratios with forest age provide a mechanistic explanation for initially lower and then gradually increasing soil emissions of the greenhouse gas nitrous oxide (N2O). The patterns of N and P cycling during secondary forest succession, demonstrated here over decadal timescales, are similar to N- and P-cycling patterns during primary succession as soils age over thousands and millions of years, thus revealing that N availability in terrestrial ecosystems is ephemeral and can be disrupted by either natural or anthropogenic disturbances at several timescales.

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These data were used to examine patterns in N cycling over the course of secondary succession and their implications for nutrient limitation of successional forests in the tropics.

Quality Assessment (Data Quality Attribute Accuracy Report):

Quality Assessment:

Lab analyses were all done according to standard protocols with calibration curves and standards included in every run.

Process Description:

Data Acquisition Materials and Methods:

Site descriptions:

Sao Francisco do Para and Capitao Poco are located in northeastern Para, southeast of the city of Belem. Mean annual precipitation is 2,200 mm for Sao Francisco do Para and Capitao Poco. The dominant soils are Typic Hapludults. Human settlement in this region expanded during the era of rubber extraction in the late nineteenth century. Government programmes for distributing rural lands for agricultural development, mostly by small landholders, increased forest clearing in the mid-twentieth century. Many farms have now undergone nine or more cycles of slash-and-burn agriculture.

Methods:

Three collections (0.25 m2 per collection) of fine litterfall were made monthly for a year in each plot at both chronosequences. Fresh foliar samples were collected in the 6-, 20- and 40-yr-old successional forests and the mature forest plots. At Sao Francisco do Para, fully expanded leaves were collected for the dominant species at each site, according to the species importance values indices. Finely ground foliar samples were analysed for C and N concentrations using a Carlo-Erba CHN analyser, for C and N stable isotope ratios using a Delta Plus ThermoQuest-Finnigan mass spectrometer, and for P by acid digestion followed by colorimetric spectrophotometry. Foliar C, N and isotope anlayses were done at CENA in Piracicaba using their standard methods (for more details see Ometto et al. 2006). Soil nutrient analyses and foliar analyses other than C, N and stable isotopes were all done at EMBRAPA Belem using their standard protocols (Silva 1999).

Soil emissions of N2O were measured at Sao Francisco do Para using syringe sampling of static chambers and gas chromatography with an electron capture detector (Verchot et al 1999). Three chamber fluxes were measured per date in each of the twenty-six plots at Sao Francisco do Para, with five dates in each of the dry and wet seasons.